This invention relates to flow sensors and more particularly to such devices for sensing failure of flow of lubricant in a lubricating system.
The invention is in the same field as and in certain respects involves an improvement upon the lubricant flow sensing valves shown in U.S. Pat. No. 3,500,958 issued Mar. 17, 1970, now owned by the assignee of this application. Each of the valves shown in this patent includes a piston constituting a valve member spring-biased toward a no-flow position in which it blocks a lateral outlet, and movable against the bias by pressure of fluid from the inlet of the valve to a flow-through position unblocking the outlet, the arrangement being such that on failure of flow the piston is spring-biased to its no-flow position and actuates means for signalling the no-flow condition. The space in the valve in back of the piston is in communication with the outlet. To retard the movement of the piston to its no-flow position, according to said patent (and as has been done in actual practice) the piston is dimensioned to have a close but nonsealing fit, referred to in said patent as a "dashpot" fit, for the purpose of signalling failure of flow only upon a relatively prolonged failure (e.g., approximately one minute) and not upon a relatively short cessation of flow. In this regard, these flow sensing valves are widely used in a lubricating system wherein lubricant is pumped from a supply by a plunger pump which is a single-acting pump operable to deliver a predetermined volume of lubricant on a pressure stroke of its plunger and to re-charge with lubricant without delivery of lubricant on a return or priming stroke of the plunger. Movement of the piston to its no-flow position (with the resultant no-flow signal) must be retarded to the extent that a no-flow signal is not given during a return stroke of the plunger in the normal course of operation of the pump, a no-flow signal being given only if flow ceases for an interval longer than the return stroke interval (and then not being too long delayed.
In actual practice, valves with the piston having the "dashpot" fit of U.S. Pat. No. 3,500,958 have presented problems in certain instances, in particular where the valves may be used under relatively high or relatively low temperature conditions. Thus, under high temperature conditions with attendant decrease in lubricant viscosity, the sensor may signal flow failure too soon, and under low temperature conditions with attendant increase in lubricant viscosity, it may signal flow failure too late. The clearance between the piston and the wall of its bore, the lubricant viscosity and the spring force on the piston are the principal factors which determine the length of time it takes the piston to move to its no-flow position. It is difficult, in manufacturing the valves, to hold close tolerance on the clearance, and since the flow through the clearance is proportional to the third power of the clearance, variations in clearance radically affect the flow and hence the time delay. Also, while the spring force is a linear variable, the viscosity of the lubricant is nonlinearly variable in accordance with its temperature. Thus, as to the prior valves shown in U.S. Pat. No. 3,500,958, it is possible to obtain consistency of the time-delay interval only over a relatively narrow range.